"Migration Watch UK are not an independent thinktank, or academic body, but a lobbying and campaigning organisation that is currently engaged in a campaign entitled 'No to 70 Million'," the letter states. "Its policy objective, described in the organisation's own words 'in a nutshell', is 'to control the number of non-EU migrants who are given the right to settle permanently in the United Kingdom'."

But the letter, passed to the Guardian, claims the BBC increasingly treats Migration Watch as "neutral analysts of UK migration patterns", adding that its opinions often go unchallenged on BBC news programmes.

"Migration Watch are not a thinktank, but a campaign group with a very clearly defined agenda," the director of Migration Matters, Atul Hatwal, told the Guardian. "No one has any problem with the BBC giving a platform to their views, but they need to be treated in the same way as any other political lobbying group. An organisation that campaigns under the slogan 'No to 70 million' and claims 'immigration will bite everywhere' clearly has a partial, rather than neutral, view of migration and its impacts."

But Sir Andrew Green, chairman of Migration Watch, insisted his group was "independent of all political parties".

"It makes no secret of its support for migration in both directions nor of its desire, shared by 70% of the public, to see a reduction in current levels of migration. It also has a track record of accuracy which is now widely acknowledged."

Migration Matters is a cross-party organisation chaired by the former Labour MP Barbara Roache and co-chaired by the Tory MP Gavin Barwell. The group's website says it has the support of "politicians, business executives and trade union leaders", adding: "We acknowledge the positives of migration, while directly confronting the challenges modern migration patterns pose to society."